1. Field of the Invention
This invention relates to a vibrating sensor for responding to physical stress, for example due to forces of pressure or acceleration.
2. Discussion of Prior Art
Sensors are known for sensing physical parameters such as pressure or acceleration in which a resonant beam is formed across a cavity in a semiconductor substrate. The beam is excited to cause it to vibrate at a predetermined frequency: the means for providing the excitation may be optical, thermal, electrostatic, electromagnetic, piezoelectric or acoustic. Stresses applied to the beam through causes such as acceleration or pressure cause the beam to change in frequency of vibration. This change in frequency can be measured and from known data the magnitude of the applied stress can be calculated.
Optically-excited vibrating beam sensors are known which utilize a Fabry-Perot interferometer to modulate light which is incident on the beam, and thereby vibrate the beam using thermal effects. The semi-reflective end of an optical fiber is disposed a precise distance from and perpendicular to a semi-reflective side surface of the beam. Light transmitted down the fiber is incident on the beam, thus heating the beam and causing it to bend. The distance between the end of the optical fiber and the semi-reflective surfaces of the beam therefore alters and the light incident on the beam is reduced. As the beam cools and returns to its rest position, the intensity of light incident on the beam once again increases: the beam is therefore alternately heated and cooled and accordingly caused to vibrate at a predetermined frequency.
In the above-described sensor, the distance between the end of the optical fiber and the semi-reflective side surface of the beam is particularly critical in order to define the Fabry-Perot interferometer. However, it is difficult to mount the optical fiber precisely enough relative to the beam. We have now devised a vibrating sensor which overcomes these drawbacks.